With continuing improvement in digital imaging technology, digital light modulation is used for a wide range of display devices such as rear projection TVs, motion picture projectors for business and entertainment markets, digital printers, and other imaging apparatus. In the development and design of such apparatus, the challenge of providing illumination having sufficient brightness is widely acknowledged. For some types of digital imaging devices in particular, such as digital projection devices, the inability to provide sufficient brightness presents a serious performance constraint. Conventional illumination solutions for digital projectors, such as UHP (Ultra-High Performance) lamps or high-pressure mercury arc lamps or Xenon arc lamps, have been employed for some digital projection systems, but are disadvantaged for a number of reasons, including short lifetimes, deterioration with age, high heat, environmentally hazardous component materials, and constrained color gamut.
Until recently, solid-state light sources such as Light Emitting Diodes (LEDs) did not exhibit high enough power levels for projection. However, high-brightness LED sources are now being commercialized and used satisfactorily in illumination systems for smaller devices, such as pocket projectors and Rear-Projection Television (RPTV) devices. When compared against conventional lamp-based illumination solutions, LEDs have some inherent advantages such as lower power consumption, longer component life, and elimination of warm-up requirements. In addition, the relative spectral purity of these sources offers the promise of a broader color gamut than is provided by conventional high-brightness lamps. LEDs also do not have environmentally hazardous materials such as the mercury contained in metal-halide lamps. LED brightness is also adjustable over a range, without changing its spectral characteristics.
As higher-brightness LEDs are being developed, there has been considerable attention directed to adapting these solid-state light sources for use in digital display and projection apparatus. For many types of display and projection designs, it is necessary to combine the light from single-color LEDs, typically Red (R), Green (G), and Blue (B) LEDs, onto a single light path, and then to direct the light to a spatial light modulator (SLM), such as a digital micromirror device (DMD) used in DLP® projection systems from Texas Instruments, Inc., Dallas, Tex.
The proposed solutions for color combination when using single-color LEDs generally use an arrangement of dichroic surfaces and light integrating components. Referring to the perspective view of FIG. 1A, there is shown an illumination apparatus 30 that employs an integrating rod 34 for mixing light from an LED assembly 32 that has two green LEDs 12g, one red LED 12r, and one blue LED 12b. Acting as a light guide that directs light by Total Internal Reflection (TIR), integrating rod 34 homogenizes the light input from the different color LEDs on LED assembly 32 to provide polychromatic light at its output 36.
The solution shown in FIG. 1A may be workable for some types of low-end display devices, but is very inefficient. As is well known to those skilled in the imaging arts, any optical system is constrained by geometrical considerations, expressed in terms of etendue or, similarly, in terms of the Lagrange invariant, a product of the acceptance solid angle and the size of the aperture at any given plane in an optical system. Where an optical system is matched and symmetric, Lagrange and etendue values are identical throughout the system. In optical systems that are not matched or symmetric, the etendue is the smallest value that allows light through the system. In the particular example of FIG. 1A, etendue-matching is relatively poor, since the output beam of light is twice as large as need be. It would be advantageous to overlap light of each of the three colors onto the same path, with an area the size of the two green LEDs 12g. 
Etendue and the corollary Lagrange invariant provide ways to quantify an intuitive principle: only so much light can be provided from an area of a certain size. As the emissive area gets smaller, the angle of emitted light gets larger in order to preserve the equivalent brightness.
Added complexity and cost result from the requirement to handle illumination at larger angles. This problem is noted and addressed for high-density Liquid Crystal on Silicon (LCOS) devices in U.S. Pat. No. 6,758,565 entitled “Projection Apparatus Using Telecentric Optics” to Cobb et al.; U.S. Pat. No. 6,808,269 entitled “Projection Apparatus Using Spatial Light Modulator” to Cobb; and U.S. Pat. No. 6,676,260 entitled “Projection Apparatus Using Spatial Light Modulator with Relay Lens and Dichroic Combiner”, to Cobb et al. These patents disclose electronic projection apparatus design using higher numerical apertures at the spatial light modulator for obtaining the necessary light while reducing angular requirements elsewhere in the system.
In display and projection apparatus, it is most desirable to match, as closely as possible, the etendue of the spatial light modulator (SLM). As a general rule, increased etendue results in a more complex and costly optical design. For a projector using the component arrangement of FIG. 1A, for example, lens components in the optical system must be designed for large etendue.
There have been a number of solutions proposed for using an integrating rod with LED sources in order to reduce etendue. For example, U.S. Pat. No. 6,956,701 entitled “Method and Apparatus for Combining Light Paths of Multiple Colored Light Sources Through a Common Integration Tunnel” to Peterson et al. describes an illumination arrangement that directs light from multiple LEDs through an integrating tunnel. Various embodiments are described in the Peterson et al. '701 disclosure for directing light from multiple LED sources into a single integrator element. However, solutions such as those proposed all tend to increase the etendue of the exiting light and can have other problems. For example, one solution proposed in the Peterson et al. '701 disclosure (FIG. 6 in the '701 disclosure) and utilized in illumination devices such as the ZoroLight™ LED Multiplexers from Bookham Display Products, Santa Rosa, Calif. can exhibit various problems that cause inefficiency and increase etendue. FIGS. 1B and 1C show how some of these problems can occur. In FIG. 1B, integrating rod 34 is a light guide formed from a solid transparent material. A green LED 12g directs light into one end of integrating rod 34. A blue LED 12b directs light into integrating rod 34 from the side. This blue light is reflected from a dichroic surface 18, internal to integrating rod 34 that folds the light paths together by transmitting green light from LED 12g and reflecting blue light from LED 12b. The preferred light path for blue LED light is shown in the solid line reflecting from the inner surface of integrating rod 34, then exiting integrating rod 34 at the right. Light loss is indicated in a dashed line labeled 38a shown exiting integrating rod 34 at an angle. This light loss can be due either to light that misses dichroic surface 18 altogether or to the imperfect performance of dichroic surface 18, which typically exhibits at least some light leakage, particularly at higher incident angles.
FIG. 1C shows an inherent problem with a related design in which integrating rod 34 is a hollow light tunnel that has a reflective inner coating. While a portion of the light reflected from dichroic surface 18 exits as intended, as shown by the solid line that exits integrating rod 34 at the right, light that does not reflect properly from dichroic surface 18 is reflected at very high angles and thus exits integrating rod 34 at very high angles, as shown by the dashed line labeled 38b. This high angle light significantly increases the etendue of the illumination system.
One solution for reducing overall etendue is to combine different color LED light sources onto the same optical path. FIGS. 2A, 2B, and 2C show conventional approaches that use various arrangements of dichroic surfaces for combining color paths or combining colors to form a polychromatic or “white” light from LED sources. FIG. 2A shows an X-cube or X-prism 10 formed from four angled prisms having dichroic coatings and fabricated as described, for example, in U.S. Pat. No. 5,098,183 entitled “Dichroic Optical Elements for Use in a Projection Type Display Apparatus” to Sonehara. X-prism 10 provides crossed dichroic surfaces 14a and 14b that are treated to direct light from a red LED 12r, green LED 12g, and blue LED 12b onto an optical axis O. Approximate light paths for each source are represented by dashed lines in FIGS. 2A, 2B, and 2C; in practice, each LED 12r, 12g, and 12b emits a cone of light and the cones of light are combined onto one path by each of these devices. There are numerous examples of color combining and separation components that employ X-cubes or X-prisms including, for example, U.S. Pat. No. 6,019,474 entitled “Modified X-Cube Arrangement for Improved Contrast Projection Display” to Doany et al. and U.S. Pat. No. 6,327,092 entitled “Cross Dichroic Prism” to Okuyama.
Another conventional color combiner is the Philips prism 20 shown in FIG. 2B. The Philips prism assembly is well known to those skilled in the digital projection arts and its use is described in a number of patents, including, for example, U.S. Pat. Nos. 4,084,180 entitled “Color Splitting Prism Assembly” to Stoffels et al., and 6,144,498 entitled “Color Separation Prism Assembly And Method For Making Same” to Bryars et al. Philips prisms have been employed as chromatic separator or combiner components in projector designs such as those disclosed in U.S. Pat. Nos. 6,280,035 and 6,172,813 (both to Tadic-Galeb et al.), U.S. Pat. No. 6,262,851 (Marshall), and U.S. Pat. No. 5,621,486 (Doany et al.), for example.
Briefly, a Philips prism assembly comprises an assembly with two triangular prisms 22 and 24 and one prism 26 that is approximately rectangular. There is an air gap 28 between the two triangular prisms 22 and 24. The rectangular prism element is optically coupled to a face of one of the triangular prisms 24, opposite the face of prism 24 that lies at air gap 28. Dichroic surfaces 14a and 14b are coated onto one face of each of the two triangular prisms 22 and 24. With this arrangement, light from red LED 12r is incident at a first surface of prism 24 and is then reflected from a second surface due to Total Internal Reflection (TIR). This directs the light to dichroic surface 14a that reflects red light onto optical axis O. Light from blue LED 12b is similarly reflected inside prism 22. Green light from LED 12g passes through both dichroic surfaces 14a and 14b. 
A third type of color combiner, shown in FIG. 2C, uses angled dichroic surfaces 14a and 14b to combine light and direct the combined polychromatic light along the direction of a common optical axis O. Surfaces 14a and 14b may be encased within a prism structure or in air. One example color combiner using this arrangement is described in U.S. Pat. No. 6,676,260 entitled “Projection Apparatus Using Spatial Light Modulator With Relay Lens And Dichroic Combiner” to Cobb et al. that describes a V-prism combiner using angled dichroic surfaces.
Conventional solutions that are patterned on the basic arrangements of FIGS. 2A-2C have been implemented for various digital display and projection devices with some measure of success. However, due to the relatively large emission angle of LEDs, which can be as much as 120 degrees, and inherent characteristic of dichroic surfaces, there are drawbacks to each solution that limit performance when used for combining color paths. As is described in detail in the '260 Cobb et al. disclosure, the spectral performance of dichroic surfaces degrades as incident light angle increases. At larger incident angles, polarization effects occur, so that the amount of light of different polarizations is not reflected equally at the same angle. This can cause color shifts in the light beam that is reflected or transmitted through the dichroic surface and can result in loss of some portion of the light. X-prism 10 designs are highly polarization-sensitive; Philips prism 20 devices are somewhat less polarization sensitive. The Philips prism solution of FIG. 2B is arranged to reduce the effects of this angular and polarization sensitivity by reducing the incident angle of light on dichroic surfaces. However, this improvement comes at the price of increased cost, size, and complexity.
Both the X-cube 10 solution of FIG. 2A and Philips prism 20 solution of FIG. 2B require relatively costly fabrication. In general, prism components and optical coatings are expensive to manufacture; with these devices, prisms must be accurately formed, then properly combined to form a single assembly. The individual prism elements must be made within narrow tolerances in order to direct each color on-axis. Any error made in prism assembly, whether resulting from the assembly method or from the tolerance variation of the individual prism elements, results in defective parts and lost value of coatings and components. The X-prism 10 is particularly vulnerable to tolerance problems, since it requires joining adjacent surfaces of four prisms having the appropriate dichroic coatings disposed at the proper angles.
For each of the color combiner arrangements shown in FIGS. 2A through 2C, the light output must be further conditioned by a light integrator for projector illumination. Where light sources are energized at separate times, the light integrator acts as a “uniformizer”, that is, provides spatial uniformity. Where light sources are combined to provide polychromatic or “white” light, the light integrator provides further mixing of the light, generally termed homogenization, so that combined polychromatic light can be utilized as a uniform white light, with substantially equal uniformity between colors at all points across the illumination beam. Pre-combining the colors using dichroic surfaces, as described with reference to FIGS. 2A-2C, at least helps to minimize the performance requirements, cost, and size of integrator elements.
Thus, it can be seen that while LEDs and related solid-state light sources hold promise for high-brightness projector illumination apparatus, there is still considerable room for improvement. Problems with conventional illumination approaches such as unwanted color shifting, polarization constraints, reduced efficiency, and poor etendue matching must be overcome in order to take advantage of these solid-state light sources where high brightness levels are needed.